Nowadays, the tight conditions for energy resources has increased the demand for crude oil and natural gas including a large amount of corrosive gas such as hydrogen sulfide and carbon dioxide, the use of which has so far been intentionally avoided.
Thus, materials to be are required to provide a higher resistance to pitting and stress corrosion cracking, in order to meet the requirement of drilling, transportation and storage in such an acidic environment.
Furthermore, the materials are required to provide a higher strength in order to meet the requirement of deeper drilling, more efficient transportation, and the reduction of drilling cost, even though a high strength steel is more susceptible to sulfide stress cracking. Therefore, higher strength steel is required to provide a higher resistance to sulfide stress cracking.
Hereinafter, we refer to stress corrosion cracking as “SCC”, and sulfide stress cracking as “SSC”, respectively, in this specification.
The following studies and proposals have been made in order to suppress pitting, SCC and SSC that may occur in a low alloy steel product such as pipes and tubes.
For suppressing pitting and SCC induced by pitting, an attempt was made to make steel without impurities. However, the techniques for minimizing the level of impurity elements and for removing inclusions using such equipment as a tundish heater, have their own limits from both the point of technique and also the cost aspects of steel making.
In order to suppress SSC, steel products have so far been improved by the metallographic method such as (1) making them with less impurities, (2) making their microstructure rich in the martensitic phase, (3) making their microstructure fine-grained, and (4) subjecting them to heat treatment for tempering at high temperatures. However, coarse nonmetallic inclusions in the steel products may cause pitting, which may often induce SSC. Thus, steel products containing coarse nonmetallic inclusions cannot be always satisfied with improvement in the above metallographic method.
Japanese Unexamined Patent Publication No. 2001-131698 pointed out that Ti carbonitride caused pitting and thus induced SSC. Most of the low alloy steel products contain Ti because Ti is often added to make them fine-grained and to increase their strength. The Ti carbonitride itself is insoluble in an acidic environment and has a high corrosion resistance and high electric conductivity. However, when immersed in an aqueous solution, it acts as cathode site to promote the corrosion of the surrounding steel matrix. The Japanese Unexamined Patent Publication pointed out that the susceptibility of pitting greatly depended on the precipitate size of Ti carbonitride, and proposed a method of suppressing pitting by reducing the content of nitrogen and removing inclusions using a tundish heater. However, this proposal is not satisfactory to suppress pitting in spite of the increased cost during steel making.
It is an objective of the present invention, which has been made in view of the above-mentioned state of the art, to provide such a low alloy steel excellent in pitting resistance that can avoid the occurrence of pitting caused by inclusions and also avoid inducing SSC.
Another objective of the present invention is to provide a manufacturing method of the low alloy steel.